In recent years, the construction and method of operation of heating and cooling apparatus have been significantly improved so as to effect a considerably higher degree of energy efficiency. Accordingly, it is becoming quite common to replace an existing older-vintage heating and cooling apparatus with such improved apparatus. When so replacing, it is highly desirable that no more wires, in addition to those already existing, be needed to connect the replacement heating and cooling apparatus with the controlling thermostat. Such connecting wires are generally located within partition walls so that having to add more wires can be difficult, especially in multi-story dwellings and dwellings with masonry interior walls.
When the existing heating and cooling apparatus is single-stage, that is, one-stage heating and one-stage cooling, there are at least four wires connecting the apparatus to the thermostat. One wire carries the heating signal, one wire carries the cooling signal, one wire carries the circulator fan signal, and one wire provides a common power return for circuits through the other three wires. When the replacement heating and cooling apparatus is also single-stage, the existing four wires are utilized and no additional wires are needed. However, when the existing heating and cooling apparatus is single-stage, either or both the replacement heating apparatus and the replacement cooling apparatus are two-stage and there exists only four connecting wires to the thermostat, a problem arises as to how to effectively control two-stage operation without adding an additional wire or wires. Typically, a thermostat controlling two-stage operation utilizes such an additional wire or wires to enable the thermostat to provide, through such additional wire or wires, the required signals for controlling second-stage operation.
In U.S. Pat. No. 4,425,930, an arrangement is disclosed wherein a two-stage heating apparatus is controlled by a conventional single-stage thermostat. It would appear that such an arrangement, when combined with a typical single-stage cooling apparatus, could enable control of the heating and cooling apparatus using only four connecting wires to the thermostat. In this prior art arrangement, the heating apparatus includes a timing means which establishes specific times in the heating cycle at which second-stage operation is to be effected. For example, second-stage operation is effected at the beginning of the heating cycle; after thirty to sixty seconds, the system operates at only the first stage; after eight to ten minutes, if the thermostat is still calling for heat, the system again operates at the second-stage. While such an arrangement may operate satisfactorily, it is believed that a more constant level of room temperature can be maintained if second-stage operation is controlled in a manner more dependent upon sensed room temperature.